Variability in the southeast Indian Ocean from altimetry: Forcing mechanisms for the Leeuwin Current

Abstract

Seasonal and interannual variability in the southeastern Indian Ocean is investigated with the aid of Topex/Poseidon (T/P) altimeter data for the 3‐year period 1993–1995. The annual Rossby wave signal around 10°S is clearly marked, and consistent with modelling and Geosat results from earlier periods. A band of higher mesoscale variability between 20° and 35°S extends across the entire Indian Ocean, with characteristic timescales between 120 and 180 days and length scales of order 500 km. Sea level anomalies are shown to propagate at around 1.5 to 2 times the theoretical linear Rossby wave speed, with an associated signal in sea surface temperature (SST) anomalies, and the propagation suggests that the variability is not locally forced but originates near the eastern boundary. Altimeter data is also used to examine variations in the alongshore pressure gradient, thought to be the principal mechanism forcing the Leeuwin Current poleward against the prevailing equatorward winds. The T/P data confirms that the alongshore pressure gradient is maximum in May when the Leeuwin Current is strongest, but we find a consistent secondary peak in November which is not evident in the climatological data. There is also significant interannual variation, related to large interannual variations on the Australian Northwest Shelf. The seasonal and interannual variations also influence the thermal structure of the instabilities associated with the Leeuwin Current, which may be the source of the westward propagating anomalies between 20°S and 35°S. The results compare well with satellite SST and in situ XBT data at the eastern boundary and suggest that satellite data can be used to monitor the variability of the Leeuwin Current.